A fuel cell has been proposed as a clean, efficient, and environmentally-responsible power source for electric vehicles and various other applications. Individual fuel cells can be stacked together in series to form a fuel cell stack for various applications. The fuel cell stack is capable of supplying a quantity of electricity sufficient to power a vehicle. In particular, the fuel cell stack has been identified as a potential alternative for the traditional internal-combustion engine used in modern automobiles.
One type of fuel cell is the polymer electrolyte membrane (PEM) fuel cell. The PEM fuel cell includes three basic components: an electrolyte membrane; and a pair of electrodes, including a cathode and an anode. The electrolyte membrane is sandwiched between the electrodes to form a membrane-electrode-assembly (MEA). The MEA is typically disposed between porous diffusion media, such as carbon fiber paper, which facilitates a delivery of reactants, such as hydrogen, to the anode and oxygen to the cathode.
In the electrochemical fuel cell reaction, the hydrogen is catalytically oxidized in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The electrons from the anode cannot pass through the electrolyte membrane, and are instead directed as an electric current to the cathode through an electrical load, such as an electric motor. The protons react with the oxygen and the electrons in the cathode to generate water.
The electrolyte membrane is typically formed from a layer of ionomer. The electrodes of the fuel cell are generally formed from a finely-divided catalyst. The catalyst may be any electrocatalyst that catalytically supports at least one of an oxidation of hydrogen or methanol, and a reduction of oxygen for the fuel cell electrochemical reaction. The catalyst is typically a precious metal such as platinum or another platinum-group metal. The catalyst is generally disposed on a carbon support, such as carbon black particles, and is dispersed in an ionomer.
The electrolyte membrane, the electrodes, the diffusion media, and a subgasket for the separation of reactant fluids are generally disposed between a pair of fuel cell plates. The pair of fuel cell plates constitutes an anode plate and a cathode plate. Each of the fuel cell plates may have a plurality of channels formed therein in an active region for distribution of the reactants and a coolant to the fuel cell. Each of the fuel cell plates may also have headers with ports and channels in a feed region for delivery of the reactants and the coolant to the active region of the fuel cell. The electrolyte membrane, the electrodes, and the diffusion media are generally disposed in the active region. The subgasket is generally coupled with the electrolyte membrane and disposed in the feed region between the pair of fuel cell plates.
The fuel cell plates are typically formed by a conventional process for shaping sheet metal such as stamping, machining, molding, or photo etching through a photolithographic mask, for example. In the case of a bipolar fuel cell plate assembly, the fuel cell plate assembly is typically formed from a pair of unipolar plates, which are then joined to form the bipolar fuel cell plate assembly. An exemplary bipolar plate is disclosed in U.S. patent application Ser. No. 11/752,993 to Newman et al., the entire disclosure of which is hereby incorporated herein by reference.
Compressive stresses are known to occur within the fuel cell stack. The compressive stresses generally occur at locations along the fuel cell plates, between which the electrolyte membrane, the electrodes, the diffusion media, and the subgasket are disposed. In particular, the compressive stresses are known to occur at the edges of the diffusion media and the subgasket. Past efforts to relieve edge stresses by using very thin subgaskets and softer diffusion media have contributed to a degraded performance of the fuel cell stack.
There is a continuing need for a fuel cell plate assembly design that desirably manages compressive stresses at edges of the diffusion media and the subgasket between a pair of the fuel cell plates.